When we adopted our Rottweiler ages ago (both the "we" and the dog are years in the past), he was 1 1/2 years old and untrained but friendly. He was also terrified of having his nails clipped. After 2 trips to the vet for this, where he got tied up and screamed like a dying horse throughout the process (plain panic, no pain), I started working with him to make it an easier job.
During play and petting sessions, I handled his feet a lot... stroking and squeezing a little and flicking my nails against his, working up to rubbing the skin between the toes and holding the paw for a short time without letting go, etc.
When he could handle that as "normal", I brought out the clippers, waited through his initial mild panic at the sight of the things, and incorporated them into the petting/playing. Stroke a foot with the clippers, click the metal against the nails, poke a little with them under the pad and toes, put them away again. Follow up with "OMG, whattaya know, you didn't die! What happened?" (it's not advisable to make a fuss of any kind, but I couldn't resist to rub it in a little. He was dense enough to not "get it", so it was ok) and a cookie
After a bunch of sessions like that, I went to holding a paw and acting as if I was clipping a nail. At this time came also the authority spiel, as in, "when I handle you, you must hold still and let me do it". All of the previous steps involved him lying down on his side. Now we worked on his rolling over on his back with his feet up. It went nicely with the authority bit.
Next, the clipping of claws. One claw per session, one cookie treat, done. After a week, 2 claws per session, one cookie per claw, done. Another week later, three claws per - you get the idea. Over the weeks, we got to one cookie per paw, and later to one cookie for all four paws (equaling one session).
It may sound like a terrible waste of time if you can just as easily manhandle a dog into submission for claw-clipping, but chances are that a manhandled dog won't flop over and stick his feet up for you for the next 10 years or so whenever you so much as click the clippers
